The inventions claimed relate generally to the cleaning of rotating filter media commonly employed in the treatment of fluids such as water, wastewater and industrial process streams. Such filters often employ textile cloth membranes of cellulose base material, other natural fibers or synthetic fibers of a specific diameter, length or cross section shape woven or napped into a tight, single layer or multiple layer fabric or matting to obtain a desired thickness and porosity. Such fibers may be needle napped to a textile support grid of the same or different materials. Such support grid is to provide lateral strength to the fabric. The fibers may also be woven into a dense cut pile fabric supported by an open weave textile support grid. The cloth filter media is stretched over large drums or multiple disk-type frames. For non-limiting examples, see U.S. Pat. Nos. 4,090,965 and 4,639,315.
Typically, the filter media is placed in the flow path of a fluid stream containing suspended solid particles which are to be removed by the filtering process. The solid particles larger than the openings in the filter media are retained on the upstream, or influent, side of the filter media while the remaining flow (the filter effluent) passes through. Over time, these solids build up into a dense film on the influent side of the filter media and impede the rate of filter effluent that passes through, thus necessitating a cleaning of the filter to remove the solids build-up. One common cleaning method known in the art is reverse flow backwashing.
Backwashing is a process whereby a small portion of the flow through the filter is momentarily reversed to slough off the concentrated surface solids and transport them to a backwash water reprocessing station. With modern filtering mechanisms of this kind, backwashing can be accomplished without taking the filter out of service or off-line.
A stationary backwash collection header or a so called "backwash shoe" is located radially against the filter media surface. On the contact side against the media there is a non-abrasive slotted orifice plate mounted to the backwash shoe. The orifice plate and the shoe are held in contact with the media by a spring loaded actuator. To avoid influent leakage into the shoe an effective orifice plate alignment has to be maintained. Leakage at this point can result in diminishing cleaning potential.
The backwash collection header (or shoe) is connected to a suction pump which actuates the backwash flow when a headloss or time set point is reached during the filtration process. During this backwash operation, the disks are rotating and the entire media surface passes across the slotted orifice openings on the stationary backwash shoe at least one time. Due to backwash fluid suction and solids mat resistance, the cloth media is partially drawn into the orifice slot and is induced to flexure which aids in separating the surface solids from the media while the back flushing is in progress. The orifice slot has certain upper and lower width limits. An orifice slot that is too small will prevent adequate backwash flow and cause possible severe bridging of solids. An orifice slot that is too wide will cause excessive "draw-in" into the orifice slot of the filter media and will tend to stretch it. Heavy "draw-in" will also increase the material wear and the drive motor power demand.
The effectiveness of the cleaning process depends on the application of a sufficient and uniformly distributed backwash flow volume through the orifice slots. Ideally the application of the backwash flow would be evenly distributed across the filter media, but inherent limitations in the current mechanical design of backwash shoes and their orientation prevent known systems from functioning in this optimized condition. Moreover, it is desirable to minimize the length and frequency of filter cleaning cycles. Consequently, a cleaning process that cleans unevenly or fails to effectively remove the collected surface solids will require more frequent cleaning and will produce less filter effluent.
It is a known problem with current methods of rotating disk filter media cleaning that the area of the rotating filter media nearest the axis of rotation is cleaned more thoroughly than the more radially distant areas. This is primarily due to the common use of a single, fixed-width orifice slot running the length of the backwash shoe. This arrangement creates a substantially higher applied backwash flow per unit of filter area at the inner portion of the rotating filter media in the prior art. It is also a known problem that uneven cleaning of rotating disk filter media reduces the rate of production of filtered effluent and increases the frequency of backwash cycles. The present inventions provide improved cleaning methods and structures that overcome these and other limitations of current backwash cleaning techniques.
A significant advantage of the present invention over prior art methods and devices for cleaning filter media is that the method and apparatus of the invention maintains the filter media in a uniformly clean, and thus, more effective filtering condition for longer periods of filter operation. This advantage results because the backwash flow rate per unit of filter area is distributed more evenly across the filter media compared to prior art filter cleaning devices and methods. The invention significantly reduces the number of wash cycles required by the filter and, thus, highly efficient filtration is achieved.
It is also a feature and advantage of the present invention to improve media cleaning performance and increase filtered water production and reduce required energy, washwater consumption, waste washwater volume and overall cost.